Melatonin

A pineal hormone acting at MT1 and MT2 receptors: MT1 damps the wake signal, MT2 shifts circadian phase. It is also a direct antioxidant that concentrates inside mitochondria, which is why a high-dose research interest exists alongside the ordinary sleep use.

Not a peptide. Large human RCT base for sleep onset.

One of the few things here with large positive human trials - and they mostly used low doses, which is the opposite of how it is usually sold.

The most repeated correction in every melatonin discussion is that people take far too much of it. The doses sold on shelves are many times what the body actually releases, and the reports of morning grogginess, headaches and "it stopped working" cluster almost entirely at the high end. The advice that keeps being given is to go down, not up. The second correction is about timing: it is a clock signal, so taking it a few hours before target bedtime is described as more effective than taking it at bedtime, particularly for anyone trying to move their sleep earlier or beat jet lag. Shift workers and long-haul travellers report the clearest benefit of any group. People expecting sedation are consistently disappointed, and are told - correctly - that they are asking it to do a job it does not do.